The present invention provides monolithic integrated circuits that are particularly adapted for use in timing applications, as well as methods for designing and fabricating same.
Conventional electronic appliances include a timing unit to provide a timing base that controls the internal functioning of the appliance as well as communications with other appliances or devices. Such timing units often comprise a resonator and a driving circuit. In many cases the resonator and circuit are fabricated using different technologies, and thus need wiring or other manner of interconnection in order to function together.
Typically, the resonator is a piezoelectric device that vibrates in free space. The most common resonator is a quartz crystal oscillator that when driven at one of its mechanical resonant frequencies has either a minimum or maximum in its electrical impedance, which is used by the driving circuit to lock itself at this frequency value. Finally, the circuit outputs a signal, for example a sine wave of a well determined and stable frequency that is used by the appliance as a time base.
Another typically used type of resonator is a “MEMS” resonator. MEMS stands for micro-electro-mechanical system. MEMS devices integrate mechanical and electronic elements on a common substrate through microfabrication technology.
For such devices, the requirement of free space vibration implies that the devices need to be carefully packaged. Otherwise they will not work as intended. For instance, quartz crystal oscillators are often packaged in hermetically sealed ceramic packages that allow motion of the quartz part. Motion in this case is driven and recorded by means of electrical connections that join electrodes on the quartz resonator with leads inside the package that in turn are connected though the walls of the ceramic package to external leads or pads. MEMS oscillators may also be placed in hermetically sealed packages. However, unlike quartz crystal oscillators, since MEMS devices are often fabricated on silicon wafers, the hermetic cavity is commonly produced on this wafer rather than on the package.
Such oscillator packaging technology requires the creation of a cavity with very well controlled conditions and is commonly filled with inert gases or a vacuum. The creation of such cavities often requires delicate and expensive assembly processes. Further, cavities pose additional challenge for making the necessary electrical connection to the oscillator devices.
Unfortunately, by their fragile nature the aforementioned free space devices are sensitive to vibration that degrades their performance or shock that can destroy them. Such free space devices are subject to a force when experiencing acceleration. This force increases as the magnitude of acceleration increases. It is possible for the force to eventually reach a level sufficient to perturb the natural motion of the oscillator and even break the delicate component.
It is also important to note that, typically, the acceleration and resulting force experienced by resonator devices is not steady, but rather changes over time. For example, in the case of a rocket launch, devices aboard the rocket experience acceleration that initially grows rapidly, reaches a maximum, and eventually returns to nearly zero as the rocket is in flight. Another example is in the case of devices located in a moving vehicle where vibrations in the vehicle translate to rapidly changing accelerations. The frequency spectrum of many common vibrations is in the range of 10 to 100 kHz. This range can be close, and often includes, the range at which common oscillators work. The fact that such devices are free to move makes them very sensitive to the aforementioned effects.
Thus, it is desirable to develop resonating devices which do not rely on free space and which can be fabricated in an integrated manner with associated circuitry such as a driving circuit. It is also desirable to optimize such resonating devices for timing applications.